I've been working on a gtksourceview2 style editor based on mike_g's C version here: http://ubuntuforums.org/showthread.php?p=5790705
By the way, Gedit uses gtksourceview2 to hilight it's code. This will create universal styles that can be used in Gedit and any others that use it as well.
I just finished 0.3 and would like to share it with anyone who wants it. It supports the main styles and diff: styles. The only gotcha is that it always saves using the File>Save menu and always saves to it's own dir as out.xml. This should be okay for now. Let me know what you think. Edit, View, and Help menus are there for looks right now. Only File>Save and File>Quit work. Try it and let me know.
You'll have to hand-edit the xml file after it's saved with your own author and title information, then copy it to ~/.gnome2/gedit/styles/ and rename it.
What else I want to do with it:
Be able to set all background colors at once.
Automatically install to ~/.gnome2/gedit/styles/
Be able to install universally with gksu
Save as... menu
Update: Version 0.4
* Save and Save As work properly now.